1. Field of the Invention
The present invention generally relates to an electret used for a condenser microphone and a method of manufacturing thereof, more specifically, to a multi-layer electret that can endure a high temperature so as to be able to be processed by surface mount technology (SMT) and has ultra-high charge stability and a method of manufacturing thereof.
2. Description of the Related Art
In general, microphones are classified, based on a method of converting mechanical vibration into electric signals, into a carbon microphone using electrical resistance characteristic of carbon grains, a crystal microphone using piezo-electricity effect of rochelle salt, a moving-coil microphone that vibrates a diaphragm having a coil in the magnetic field to generate induced current, a velocity microphone that uses a phenomenon that induced current is created when a metal leaf set within the magnetic field receives a sound wave to vibrate, and a condenser microphone that uses a variation in capacitance caused by vibration of a film by a sound wave.
The condenser type is widely used as a small-size microphone. However, this needs DC power (battery) for applying a voltage to a condenser. To solve this problem, an electret condenser microphone using an electret having quasi-permanent charges has been recently employed. Since the electret microphone does not require a bias voltage, its pre-amplifier is simplified and its performance can be improved with low costs.
Meanwhile, with development of manufacturing techniques of electronic products, all the products are becoming small-sized. To manufacture these small-size products, surface mount technology (SMT) is widely used. The SMT is a process or a system that places components on a printed circuit board (PCB). The surface-mounted component have very small leads so as to be connected (welded) to lands provided on the PCB or do not have the leads. Although the price and performance of a product can be improved when the SMT is applied to manufacture the product, the SMT cannot be applied to components that are vulnerable to a high temperature because a high temperature is applied to them during reflow process.
A conventional electret used for a condenser microphone is made of fluoro-Ethylene-Propylene (FEP), as shown in FIG. 1. However, FEP is vulnerable to a high temperature so that SMT cannot be used for manufacturing the electret. Therefore, production costs of various products using the microphone, for example, a cellular phone terminal, cannot be reduced. Specifically, the conventional electret 16 is made of an FEP film 12 that is laminated on a metal plate 11, and a back electret structure is constructed by forming a space with a diaphragm 17 configured in a manner that a metal 15 is coated on a PET 14 and a spacer 13. The FEP film 12 is formed by melting-extrusion so that there are limitations in increasing crystallinity thereof. In addition, among physical characteristics of the material of the FEP film, the melting point is 260xc2x0 C. approximately. It means that it becomes liquid at a low temperature. Accordingly, it is difficult to manufacture the electret that is processed by SMT using the FEP film.
To solve the above-mentioned problem, SiO2, Si3N4, and the like have been developed as materials of electrets that can endure a high temperature as a result of steady research and development. However, a manufacturing process of electrets using these materials is complicated and manufacturing cost is high because they are ceramic materials. Furthermore, a charging process, which is a process of converting a dielectric into electrets, is difficult to perform because of high density of ceramic material. Even if the electrets are charged, only their surfaces are charged so that the quantity of electric charges are decreased very easily when humidity or other impurities come into contact with the surfaces of the electrets. Therefore, the above-mentioned materials have a problem that they are vulnerable to moisture or surrounding environments though they can endure heat.
It is, therefore, an object of the present invention to provide a polymer electret having ultra-high charge stability that can endure a high temperature and humidity.
Another object of the present invention is to provide a method of manufacturing the above-mentioned electret for SMT.
To accomplish the objects of the present invention, there is provided a multi-layer electret that is constructed in such a manner that an FEP film of 12.5 xcexcmxcx9c25 xcexcm in thickness melting-adheres to a surface of a metal plate, and a PTFE film of 30 xcexcmxcx9c100 xcexcm in thickness melting-adheres to the surface of the FEP film.
The present invention further provides a method of manufacturing a multi-layer electret having ultra-high charge stability, comprising a first step of laminating an FEP film on a metal plate; a second step of heating the metal plate on which the FEP film was laminated at a high temperature and applying high pressure thereto, whereby the FEP film and the metal plate melting-adhere to each other; a third step of laminating a PTFE film on the FEP film attached to the metal plate; a fourth step of heating the laminated structure and applying high pressure thereto, whereby the PTFE film melting-adheres to the FEP film; a fifth step of cooling the resulting structure, to accomplish an electret; and a sixth step of charging the cooled electret with charges.